fix(helcim): read dateBilling on subscription CREATE to populate next-billing cache

Helcim returns next-charge as `dateBilling` on POST /subscriptions, but the
two CREATE sites were reading `subscription.nextBillingDate`, leaving
`member.nextBillingDate` empty after every signup and free→paid upgrade.
The lazy refresh in subscription.get.js (which already accepts both shapes)
masked it on next account-page load, so renders eventually populated — but
the success response we returned to the client also had `nextBillingDate:
undefined`. Mirror the GET-side resolution at both CREATE sites: prefer
`dateBilling`, fall back to `nextBillingDate`. Existing Number.isNaN guard
unchanged; defensively rejects malformed strings from either field.

server/api/members/update-contribution.post.js

@@ -100,9 +100,8 @@ export default defineEventHandler(async (event) => {
           throw new Error("No subscription returned in response");
         }
 
-        const nextBillingDate = subscription.nextBillingDate
-          ? new Date(subscription.nextBillingDate)
-          : null;
+        const rawNextBilling = subscription.dateBilling || subscription.nextBillingDate || null;
+        const nextBillingDate = rawNextBilling ? new Date(rawNextBilling) : null;
 
         // Update member record
         await Member.findByIdAndUpdate(
@@ -149,7 +148,7 @@ export default defineEventHandler(async (event) => {
           subscription: {
             subscriptionId: subscription.id,
             status: subscription.status,
-            nextBillingDate: subscription.nextBillingDate,
+            nextBillingDate: rawNextBilling,
           },
         };
       } catch (error) {